1. Technical Field of the Invention
The invention relates generally to the field of integrated circuit and, more particularly, to fusible link programming in semiconductor integrated circuits.
2. Description of Related Art
In integrated circuits including CMOS integrated circuits, it is often desirable to be able to permanently store information, or to form permanent connections of the integrated circuit after it is manufactured. Fuse or anti-fuse devices forming fusible links are frequently used for this purpose. Fuses and anti-fuses can also be used to program redundant elements to replace identical defective elements such as DRAM, Flash EEPROM, SRAM, or other memories. Further, fuses can be used to store die identification or other such information, or to adjust the speed of a circuit by adjusting the resistance of the current path.
One type of fuse device is “programmed” or “blown” using a laser to open a link after a semiconductor device is processed and passivated. This type of fuse device requires precise alignment of the laser on the fuse device to avoid destroying neighboring devices. This and other similar approaches can result in damage to the device passivation layer, and thus, lead to reliability concerns. For example, the process of programming the fuse can cause a hole in the passivation layer when the fuse material is displaced. Also the method is not in-system, sometimes inconvenient, and thus lead to higher test cost.
Another type of fuse device is the electrical fuse/anti-fuse. Electrical fuse/anti-fuses have been introduced into semiconductor products and are, in many applications, replacing the commonly used laser fuses. The typical electrical fuse/anti-fuse is in-system but is one-time programmable. It is generally a passive element such as resistor or capacitor which is programmed or blown using electrical pulses via a programming (pass gate) transistor. Since significant energy or high programming current is required to pass through these devices to reach the passive element, the size required for the programming (pass gate) transistors can be very large.
For example, a currently used anti-fuse device is structured based on a conventional MOS transistor. Such an anti-fuse is programmed by applying a voltage (generally about 7 Volts) across the gate-oxide of the MOS transistor. The programming process results in a damaged gate-oxide which reduces the electrical resistance across the oxide. A sensing circuit attached to the anti-fuse is used to differentiate between the high resistance of the intact oxide and the lowered resistance of the damaged oxide. For lower resistances and more reliable sensing, even higher programming voltages and programming currents are used.
Because of the significant energy required for programming, damage can result to surrounding structure and/or unreliable sensing can result because of the inconsistent nature of the blow process and the relatively small change typically offered in the programmed resistance. Further, these type of devices may not be viable for use with many of the latest process technologies because of the required programming potentials, i.e. high current flow and high voltage levels over a requisite amount of time. It would be advantageous to lower the programming parameters in order to enable reduction of the size of the associated circuits (e.g. voltage generator, programming transistor, wiring, etc.) and/or improve the sensing reliability.